The Taliban have warned of "consequences" if the United States fails to meet a deadline to withdraw its troops from Afghanistan, after President Joe Biden said doing so would be "tough".

The United States is supposed to complete a full withdrawal of troops within six weeks, according to an agreement signed in Qatar with the Taliban.

Biden on Wednesday cast doubt on meeting the deadline, saying he was in the process of deciding when troops would leave.

"The Americans should end their occupation in accordance with the Doha deal and fully withdraw their forces from Afghanistan by May 1," Taliban spokesman Zabihullah Mujahid told AFP.

"If they don't do it, be it for any reason and pretexts, then they will be responsible for the consequences," he said, adding that "the people of Afghanistan will make their decision".

The deal, which excluded the Afghan government, saw the Taliban pledge in return to hold peace talks with the administration of President Ashraf Ghani.

The talks however have made little progress since starting in September, while violence -- which Washington has blamed on the Taliban -- has surged.